Contemporary Ergonomics and Human Factors 2013

PREFERRED OR ADOPTED TIME HEADWAY?

A DRIVING SIMULATOR STUDY

Magali Gouy

1

, Cyriel Diels

2

, Nick Reed

1

,

Alan Stevens

1

& Gary Burnett

3

1

TRL, UK

2

Coventry School of Art and Design, UK

3

University of Nottingham, UK

The aim of this study was to disentangle two concepts that are

often confounded in driving and ergonomics research: preferred and

adopted time headway (THW). It is argued that preferred THW rep-

resents a range of THWs drivers feel safe and comfortable with. In

contrast, the adopted THW is the THW that drivers indeed adopt

in a certain driving situation, which depends on current situational

factors (e.g. visibility, other drivers, traffic flow). Situational fac-

tors were varied in a simulator study, as participants were asked to

follow a lead vehicle in three different traffic conditions. The pre-

ferred THW was assessed after each traffic condition by means of

the psychophysical method of limits. Preferred and adopted THW

were compared in the data analysis and results support the existence

of two distinct constructs.

Introduction

When following a car, drivers are constrained by the speed of a lead vehicle (LV) in

contrast to free driving where drivers are not constrained in this way (Vogel, 2002).

Time headway (THW) is a commonly used parameter to estimate the criticality of

a car-following situation and is defined as the elapsed time between the back of the

LV passing a point on the roadway and the front of the following vehicle passing

the same point (Evans, 1991). The adopted THW is an outcome of the interaction

between individual characteristics and situational factors.

On the one hand, individual characteristics appear to have a persistent, durable

influence on THWs. It has been reported in the literature that the “preferred” THW

is related to drivers’braking performance and perceptual-motor skills (vanWinsum,

1998; van Winsum & Brouwer, 1997; Van Winsum & Heino, 1996). Thus, drivers

who maintained a shorter THW were more efficient in their control of braking,

braked harder and adjusted the intensity of braking better to the criticality (as

measured by time-to-collision, TTC) of the moment the LV started to decelerate

compared with drivers who preferred longer THWs. Further, it is often reported in

literature that personality factors such as sensation-seeking influence the choice of

THW, whereby individuals scoring high on sensation-seeking tend to leave shorter

gaps (for review see Jonah, 1997). Finally, individualdifferenceshave been reported

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